1. Field of the Invention
The present invention relates to a vibration wave driving device for obtaining a driving force by causing a vibration member to produce a vibration wave.
2. Related Background Art
The outline of the principle of a vibration motor using a travelling vibration wave is as follows. A vibration member (stator) is manufactured by fixing two groups of piezoelectric elements arranged in the circumferential direction to one surface of an elastic member formed into a ring-like shape and made of an elastic material such as a metal whose total length is an integral multiple of a length .lambda..
These piezoelectric elements are arranged so as to alternately have opposite expansion-shrinkage polarities at a pitch of .lambda./2 in each group. Also, the piezoelectric elements are arranged so that there is a difference of an odd-number multiple of .lambda./4 between the two groups. Electrode films are formed on the two groups of the piezoelectric elements.
When an AC voltage is applied to only one group (to be referred to as phase A hereinafter), a standing wave (wavelength .lambda.) of a bending vibration, in which the central point of each piezoelectric element of the phase A and points at intervals of .lambda./2 from the central point are antinodes and the central points between these antinodes are nodes, is produced throughout the entire circumference of the elastic member of this vibration member.
When an AC voltage is applied to the other group (to be referred to as phase B hereinafter), a standing wave is similarly produced. However, the positions of antinodes and nodes are shifted .lambda./4 from those in the standing wave of the phase A.
When alternating signals having the same frequency and a timewise phase difference of 90.degree. are simultaneously applied to phases A and B, the standing waves of the two phases are synthesized. Consequently, a travelling wave (wavelength .lambda.) of a bending vibration which vibrates in the circumferential direction is produced in the elastic member. At this time, each point in the elastic member having a thickness shows an elliptic motion.
Accordingly, when a ring-like movable member (rotor), for example, is directly brought into tight contact with one surface of the elastic member, this movable member is rotated by a frictional force in the circumferential direction from the elastic member. Also, to increase the circumferential component of the elliptic motion, a plurality of radial grooves can be formed in the circumferential direction in the surface of the vibration member away from the surface to which the piezoelectric elements are fixed. As a consequence, the neutral plane of the vibration moves to the piezoelectric element fixed surface, and this increases the rotating speed for the same amplitude of an input alternating signal. This also effectively increases the motor efficiency.
A vibration wave motor as a vibration wave driving device based on the above principle has a low-speed high-torque motor characteristic and is therefore suited to high-accuracy rotation or high-accuracy positioning. Also, a vibration wave motor performs driving with a frictional force, so a larger frictional coefficient is more desirable in terms of motor performance.
One drawback of a vibration wave motor is that the sliding material used in the tight contact portion easily abrades. Therefore, it is desirable that the sliding material have a high abrasion resistance and a high thermal conductivity and readily dissipate frictional heat having an adverse effect on the abrasion resistance, and more importantly, the sliding material be readily obtainable in practice, inexpensive, and processable at a low cost.
Accordingly, the general approach is to harden the surface of an aluminum alloy or form a plating containing ceramic particles. Additionally, a silicon (Si)-containing aluminum-based alloy having a relatively high abrasion resistance among other aluminum alloys is proposed as the sliding material of a vibration wave motor.
As a counterpart material of this sliding material, a ceramic harder than usual metals or a resin material which forms a thin film on the sliding surface of the sliding material and gives lubricating properties to the sliding material is currently proposed for use in a vibration wave motor, since each material has a relatively high abrasion resistance and changes the abrasion surface little over long time periods.